HP Classic Model LED Display Information

by Alex Knight, updated October 29, 1997. Also contributed to by Joe Rigdon.

The original HP Classic-series models share a common display technology. All of the Classic machines and the HP46 and HP81 desktop machines have 15-digit LED displays which are made up of three, five-digit display modules. While LED displays are generally robust and long-lived, several of the Classic HPs I've come across have had either a dead or weak display segment which required replacing one or more of the display modules. I've gathered the following info. from several sources in an attempt to understand how these displays work.

Counting from the left, digit 1 of the display only displayed the sign (nothing for positive, middle segment illuminated for negative). Digits 2-12 displayed the mantissa and the decimal point. The Classic HPs were somewhat unique because they dedicated a full LED digit position for the decimal point. This added somewhat to the cost since LEDs were expensive back then, but made the display more readable and provided a convenient way to indicate a low battery condition, which was done by lighting all of the decimal point indicators across the display. Even with all decimal points lit, the user could figure out where the actual decimal point was because of the use of an entire digit position. Digit 13 displayed the sign of the exponent, again using only the middle segment. Digits 14 and 15 were used for the exponent.

The part number for the 5-digit LED display modules that I've pulled from several machines built between 1973 and 1975 is HP# 1990-0335, but this number does not appear in HP's Optoelectronics catalog. The good news is that there is a drop-in substitute, HP# 5082-7405 that is still available although somewhat expensive.

The 5-digit LED module comes in a standard 14-pin Dual Inline Package (DIP) type of IC packaging, which is nominally 0.300 inches wide with 0.100 inch pin spacing. The package itself is nominally 0.755 inches long. The LEDs themselves are encapulated within a red plastic material that makes up the package body. The LED digits are nominally 0.11 inches tall and 0.062 inches wide. By looking carefully, one can observe the lead frame and the tiny bonding wires that connect the lead frame to the LED segments. Molded into the top of the package are magnifying bubbles, one for each digit position. HP put a lot of thought into the geometry of the magnifiying bubbles and the encapsulating material, which manifests itself by having the best viewing angle and contrast for LED displays that I've seen from any manufacturer.

HP uses a highly multiplexed arrangement for driving the LED displays. From a power management point of view, which was important for a battery-powered calculator, LEDs are much more efficient from a brightness/power ratio when they are multiplexed vs. when they are driven all the time by DC current. Multiplexing also greatly simplifies the I/O connectivity between the displays and the controller ICs. Joe Rigdon reports that a LED segment is actually on about 1% of the time using this scheme.

The LED segments of each common segment position within the display have their anodes connected together to a common input pin, and the cathodes for all of the LED segments within a digit position are connected together to a common input pin. This is illustrated through the following ASCII diagram and pinout:

  |         |
 f|         |b
  |    g    |
  |         |
 e|    dp   |c
  |    #    |

Pin #    Function
-----    --------
  1    CATHODE - digit 1
  2    ANODE - segment e
  3    ANODE - segment c
  4    CATHODE - digit 3
  5    ANODE - decimal point dp
  6    ANODE - segment d
  7    CATHODE - digit 5
  8    ANODE - segment g
  9    CATHODE - digit 4
 10    ANODE - segment f
 11    unconnected
 12    ANODE - segment b
 13    CATHODE - digit 2
 14    ANODE - digit a

The HP Classic handheld models that I've seen have three LED display modules mounted at the top of the keyboard printed circuit board (PCB). Above the modules are inductors that are used to store a charge into and then the charge is dumped into the displays to increase the brightness. Under the display modules are two 20 pin ICs on all of the Classics except the HP67 - the anode driver is on the left and the cathode driver is on the right. These ICs scan sequentially scan through the digit positions, lighting the appropriate LED segments by activating the anodes while the cathode is activated for the particular digit. For instance, to light a "7" in digit position 3, the anode driver IC would drive pins 3, 12, and 14 while the cathode driver IC would drive pin 4 of the display module. The multiplexing is done so fast that the human eye doesn't detect it under normal room light.

Joe Rigdon has observed HP Part Number 1820-1029 for the anode driver IC and HP Part Number 1820-1061 for the cathode driver IC in HP35s, HP80s, HP81s, and one or both of these in HP45s. I have observed -1029 for the anode driver IC and 1820-1226 for the cathode driver in a HP55. The HP67 has a 1820-1749 for the anode driver, with a 16-pin cathode driver IC from RCA that has a HP Part Number of 1858-0050-1D.

The anode/cathode driver ICs are also involved in keyboard scanning and in indicating the low battery condition for the calculator. Joe Rigdon reports that the anode driver IC also generates 2 clock signals that operate the logic in the rest of the machine, and that it's common for this circuit to fail and for the calc to die. The clock signals are routed from the display PCB to a clock driver IC on the main PCB through the connector fingers - a failure is possible here as well. The symptoms of an anode driver failure are that the calc will do nothing or it may flicker a few display segments when you first turn it on. Occasionally random segments will stay on but there will be no response from the keyboard. It should be noted that similar symptoms can be due to a dirty/faulty power switch, and in case of a dead machine this should be checked first.

The HP46 and HP81 desktop calculators use a similar arrangement for the display modules. The HP46, introduced at roughly the same time as the HP45 handheld machine, is primarily a printing calculator, but HP offered a display option (Opt. 001) for an additional $100. The same type of LED modules and driver ICs as used in the handheld models are mounted on a PCB that simply plugs into an edge-card connector on the machine's main board. The PCB is required whether or not the LED modules are present because the driver ICs on the PCB provide clock signals to the main board.

Troubleshooting Classic model display problems

The quickest way to determine if a Classic model has a display problem is to press "CLx", make sure it displays "0" or "0.00" (depending on model) then press the decimal point to make sure the left-most DP lights. Press "CLx" again, then the "8" key 10 times, watching with each press to make sure all the segments light and the decimal point lights in each position. Then press "CHS" to make sure the mantissa minus sign lights. Then press "EEX", "8" 8" "CHS" to make sure the exponent digits and sign all light up correctly. The above procedure tests to make sure all the LED segments will light, but it is also possible to have other problems. Joe Rigdon has indicated that it is very common for segments to become shorted together (such as a "4" appearing as a "H"), and recommends to also try entering "1234567890" and similar combinations to make sure there are no other problems.

If a single segment or two are dead or weak, then the problem is likely in the LED module itself. If an entire digit is dead, then the problem could be in the LED module, the cathode driver IC, or possibly but unlikely a break in the PCB or a bad solder joint. If the same segment is dead across all 5 digits in a LED module, then the problem could be in the module, the anode driver IC, or possibly but unlikely a break in the PCB or a bad solder joint.

Of the likely scenerios, the "best case" for having a chance to fix the machine is having a problem with the LED module, because they are still available from HP Optoelectronics. The anode and cathode driver ICs were specialized to the Classic series machines and the supply of spare parts, from what I hear, is extremely limited and they may have to scavanged from other machines that are broken for a different reason. In mid-1997, Arrow Electronics, a major components distributor, still had a supply of the HP# 5082-7405 LED modules, at a price of approx. $25.00 U.S. each. That's the good news. The bad news is that there is a significant variation on the brightness of different lots of the LED modules, and each module is marked on the bottom with a letter indicating the relative brightness. Some modules I obtained that had a date code of late 1996 were marked with the letter "W" and were significantly brighter than the modules I had pulled from a 1973-vintage machine that had modules marked "J", which were themselves slightly brighter than modules marked "H" from a 1975- vintage machine. Therefore, if you decide to have a module replaced, you will likely either have to live with having a group of 5 digits at a different brightness, or to buy three brand new modules at a cost of about $75.00.

Should you decide to have a display module replaced, I absolutely, positively, without a doubt recommend that you do not attempt it yourself, as this is a very, very difficult thing to do on a valuable Classic machine without messing something up. If you must have a LED module replaced, be sure to get someone who has experience at doing this and who has the proper tools and equipment. The Classic machines are very difficult to work on for several reasons. The display/keyboard PCB itself is very, very thin and it would be easy to crack the PCB or to rip one or more of the copper tracks right off the board. On some of the Classic models there are other components crammed close by the display modules, which makes desoldering and working around the modules very difficult. The plated-through holes in the PCB that the modules are soldered into are very small and a tight fit with the leads of the LED modules. This makes it nearly impossible to use standard desoldering techniques to get the solder out of the holes so that the modules can be removed without causing physical damage to the module or to the PCB. I don't believe that either solder wick or a hand-held solder-sucker would work well enough to for successfully removing a module.

Even if you decide to remove a bad module, getting a new one installed correctly isn't a piece of cake. The main reason is that the LED modules are not mounted flush on the PCB, they are mounted at an angle. Pins 1-7 are soldered into the PCB as far as they will go, but pins 8-14 are bent in such a way to provide an angle for the display. It is very hard to manually get all three modules to line up perfectly. The alignment should be checked very, very carefully before buttoning up the machine.

I hope this information has been helpful regarding the display technology used on the Classic HP machines. A future update will have more information about the display driver ICs used on other Classic models.

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